The Dynamic Atmosphere Study Guide
The earth is a round ball in space. It is bathed by vast quantities of rays of sunlight. These two facts create differences in the amount of sun received by various parts of the planet, and these differences, in turn, set up whirls and swirls in the air at the global scale. Mountain ranges, continental shapes, and the contrast between water and land also contribute to the circulation patterns we experience every day as parts of our dynamic atmosphere.
The Global Atmospheric Whirls
The most important fact about the sun, other than its stupendous incoming energy, is the geometry of its rays. The rays of energy that come from the sun across space to fall upon Earth travel in nearly perfectly parallel alignment.
The most important geometric fact about the earth is that it is almost a perfect sphere. Imagine holding a soccer ball in the sunlight. The light is brightest at the place on the ball where the sunlight falls perpendicular to the surface of the ball. Then the degree of illumination fades as you look at spots on the ball further and further away from the brightest point.
We have already seen how the seasons are caused by the amount of sun received by different latitudes at various times of the year. We have also seen how various zones of climate are established, from equator to pole, by the differences in the amount of solar energy received by the zones. Now we will look into how the differences in heating influence the dynamics of the atmosphere.
Molecules of hot air move faster than molecules of cold air move. Wherever warm and cold are in con tact, the difference drives a transport of energy. We hear and see this transport as wind, which rustles leaves, pushes waves, and sweeps clouds across the sky. The most important large-scale wind pattern on Earth is caused by the tendency of hot air to rise.
The largest region of hot air on the planet is found in the tropics, the region that borders both sides of the equator. In the tropics, warm buoyant air ascends upward into the high troposphere. The rising air cannot descend back to exactly where it came from because right behind it is more rising air. Thus, at the top of its rise, it pushes out to the north and south, horizontally. Eventually, this air descends, at about latitudes 30° north and south. As we will see in the next lesson, this has implications for the location of some of the world's great deserts. For now, we note that the air returns toward the equator in the lower troposphere, creating a cycle.
Review this cycle: rising air at the equatorial topics, movement in the upper troposphere toward both north and south, then descent around 30° north and south, and finally movement in the lower troposphere back toward the equator. This pattern is named the Hadley cell, after its discoverer and is the most important large-scale atmospheric pattern.
We next consider why the lowermost, return portion of the Hadley cell— for example, the southward air flow from 30° N back to the equator—occurs not along a line of longitude but deflected toward the west to become what is known as an easterly wind. (Winds are named by the direction from which they blow.) This deflection is caused by what is called the Coriolis effect. This is sometimes called the Coriolis force, but it is not really a force in the way the word is used in physics.
The Coriolis effect (again, named after its discoverer) comes about because the earth is spinning. Everything at a given locale on the planet has a velocity of that locale. For example, if you happen to be near the latitude of New York City (about 40° N), you have an eastward velocity from the earth's spin of 740 miles per hour. If you were at the equator, your local east ward velocity imparted by Earth's spin would be 1,050 miles per hour. Why the difference? Both locations—New York City and a point on the equator—make a complete cycle in 24 hours. The point on the equator travels much farther because it goes around Earth's entire circumference. New York City travels a line of latitude where the circle is much smaller than Earth's circumference at the equator.
Now consider the return flow of the Hadley cell, which moves from 30° N back toward the equator. The eastward velocity from Earth's spin that the air has at 30° N is less than the eastward velocity from Earth's spin of the air at the equator. So as the air from the north moves southward toward the equator, as part of the Hadley circulation, it encounters air that is moving faster eastward. It tends to lag behind, deflected in a relative sense, westward. This westward deflection turns it into what is called an easterly wind.
The same logic can be applied to the Southern Hemisphere. Wind moving northward from 30° S toward the equator will be deflected toward the west, because it lags behind the equatorial air that is traveling faster eastward. So the situation is the same on both sides of the equator. In conclusion, the combination of the Hadley cell circulation and the Coriolis effect imparted by the spinning Earth creates the famous easterly winds of the tropics. These easterlies are called the Trade Winds, because commercial sailing ships of centuries ago used them as reliable sources of propulsion with a known direction.
In the higher latitudes, more circulation cells are formed by the dynamics of the sun on the round earth and by its interaction with the dominant presence of the Hadley cell. For most of the readers of this book who live in the midlatitudes, say from 30 to 60° N, the most important fact of atmospheric circulation are the dominant westerlies.
The westerlies of the midlatitudes are also created by the Coriolis effect. In the midlatitudes, a northward flow of air happens as part of the lower troposphere portion of another large-scale system of ascending and descending air (see Figure 10.1 below). The air coming from the south has a faster eastward velocity because of Earth's spin than does the air farther to the north that it encounters. This is just the opposite of the situation described earlier with the Hadley cell's return flow. Here, in the midlatitudes, the air with the faster eastward speed gets ahead of the air it encounters further to the north, which has a slower eastward speed from Earth's spin. The northward moving air is deflected to the east, relative to the air it encounters.
Deflection to the east creates a westerly wind, using the standard terminology for naming winds. Thus, in the midlatitudes, the dominant winds are westerly. That is why the weather patterns in Chicago, say, often reach New York City in the next day or two.
Study Figure 10.1, then move on to the practice questions. Be aware that all these wind patterns are formed as part of the tendency of Earth's atmosphere to even out the differential heating of the planet, caused by the rays of sunlight falling unevenly on a sphere.
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